NAT L INST. OF STAND & TECH
NISI
PUBLICATIONS
Test Proceduresfor DevelopingSolder Data
T.A. Siewert
C.A. Handwerker
QC100 nist
National Institute of
Standards and Technology
Technology Administration
U.S. Department of Commerce
NIST Recommended Practice Guide
Special Publication 960-8
Test Proceduresfor DevelopingSolder Data
NEMI Task Group onLead-Free Alloys andReliability
Edited by T.A. Siewert
and C.A. Handwerker
Materials Science and
Engineering Laboratory
August 2002
U.S. Department of CommerceDonald L. Evans, Secretary
Technology Administration
Phillip J. Bond, Under Secretary for Technology
National Institute of Standards and TechnologyArden L. Bement, Jr., Director
Certain commercial entities, equipment, or materials may be identified in
this document in order to describe an experimental procedure or concept
adequately. Such identification is not intended to imply recommendation or
endorsement by the National Institute of Standards and Technology, nor is it
intended to imply that the entities, materials, or equipment are necessarily the
best available for the purpose.
National Institute of Standards and Technology
Special Publication 960-8
Natl. Inst. Stand. Technol.
Spec. Publ. 960-8
40 pages (August 2002)
CODEN: NSPUE2
U.S. GOVERNMENT PRINTING OFFICEWASHINGTON: 2003
For sale by the Superintendent of Documents
U.S. Government Printing Office
Internet: bookstore.gpo.gov Phone: (202)512-1800 Fax: (202)512-2250
Mail: Stop SSOP, Washington, DC 20402-0001
FOREWORD
This publication documents standardized test procedures that can produce
valid and reproducible mechanical-property data for lead-free solders.
Such data speeds the application of lead-free solders in high-volume,
automated production of electronic assemblies, especially when current
production expectations combine high levels of quality with the lowest cost.
Use of standardized procedures facilitates the comparison of data between
laboratories and permits the combination of data from different sources into
a single, comprehensive database.
Most dimensions and temperatures are listed without tolerances. Unless
otherwise specified, use ±5% on dimensions, times, and pressures, and
± 3 °C for temperatures.
Many of the procedures assume some skill in the arts of specimen production
and testing. Various textbooks and industry brochures (some listed in the
Bibliography) can provide background information on these skills and any
hazards associated with these procedures.
Solder Database
Various researchers have already generated useful data; often with procedures
similar to those listed above. These data have been collected and are available
on the web at www.boulder.nist.gov/div853/. If you have data to offer,
please send it to Tom Siewert at [email protected].
iii
Acknowledgments
ACKNOWLEDGMENTS
A primary source of test methods in this document is the 1998 NCMSLead-Free Solder Project CDROM (available from NCMS). Members of
the NEMI Task Force on Lead-Free Assembly participated in
the review and refinement of these procedures.
v
Table of Contents
TABLE OF CONTENTS
Solder Database /'//
List of Figures viii
Introduction 1
0.0 Materials and Processing Details 3
I. Thermal Analysis 4
2.0 Wetting Balance Test 5
3.0 Tensile Specimen Fabrication 6
4.0 Tensile Test Procedure 8
5.0 Creep Test Procedure 9
6.0 Metallography 10
7.0 Measurement of Solid State Intermetallic
Compound Growth Rate (by dip method) 11
8.0 Measurement of Liquid-State Intermetallic
Compound Growth Rate (by the hot plate method) 14
9.0 Fracture Toughness 17
10.0 Thermomechanical Fatigue Testing —Simulated Solder Joints 19
II. Thermomechanical Fatigue Test Using PWB Test
Vehicles 21
12.0 Thermomechanical Fatigue Test Using Strips 22
13.0 Liquid Solder Dissolution 23
14.0 Acoustic Measurements of Elastic Constants 26
15.0 Ring-and-Plug Test 27
16.0 Stress-Rupture Test 29
17.0 Constitutive Behavior 30
Bibliography 31
vii
T*, c ""'<; oping Solder Data
List of Figures
Figure 1. A typical lead-free solder joint structure after reflow.
This SEM micrograph (1800x) shows the (Ni,Cu)-Sn
intermetallic reaction layer between Ni and solder and
Ag-Sn intermetallic rods crystallized from Sn-Ag-Cu
solder. Include a magnification marker on the image,
so the magnification calibration does not change with
enlargement in printing 13
viii
Introduction
INTRODUCTION
Around the world, electronics manufacturers are considering using alternatives
to the traditional lead-containing solder alloys. Unfortunately, there are much
less data available for most of these lead-free alloys, and what data exist
have often been generated with a variety of procedures. Thus, laboratories
continue to develop the mechanical-property data needed by their customers;
sometimes having to replicate the experiments of others because of differences
in procedures. The continual development of new solder fluxes and processing
technology also drives the production of additional data. Broad adoption of
standard procedures could greatly reduce the number of experiments that
would otherwise have to be performed.
The procedures described in this publication have been shown to produce
data that are applicable to the design and control of high-volume, automated
production of electronic assemblies. Use of standardized procedures facilitates
the comparison of data among laboratories and permits the compilation of data
from different sources into a single, comprehensive database.
The following pages describe the wide variety of mechanical property tests
that were selected for this collection, then refined, by the NEMI Task Groups
on Lead-free Alloys and Reliability. Use as many of the tests as needed for
your application. When you perform any of these tests, record as muchinformation as possible. In the Foreword, we include a link to some data that
have already been generated and entered into a prototype public database.
Additional data are welcome.
1
Material and Processing Details
0.0 Materials and Processing Details
Unique identifiers to be reported for the following tests.
0.1 Source of Materials
Batch or lot identification (substrate, solder and flux, as appropriate).
0.2 Specimen Identification
Type, orientation, dimensions.
0.3 Composition
Major elements, any information on trace elements; analysis method.
0.4 Flux (if used)
Standard type (e.g., standard activated rosin flux described in
ANSI/IPC/EIA Standard J-STD-002B), composition or product ID.
0.5 Substrate (if used)
Part number, structure, thickness of layers, purity, plate or wrought, etc.
3
es for Developing Solder Data
1.0 Thermal Analysis
Evaluate melting and solidification temperatures.
1.1 Equipment
Differential Scanning Calorimeter (DSC) or Differential Thermal
Analyzer (DTA).
1.2 Specimens
About 10 mg.
1.3 Parameters
Ramp rate: 5.0°C/min:
Scan: (Scan 3 times, but discard the first scan):
Nitrogen purge.
1.4 Evaluation
Measure peak and onset temperatures of each peak through both heating
and cooling.
Note: The 5.0°C/min cooling rate allows accurate measurement of the
melting and solidification temperatures of these alloys, but this cooling rate
is too slow to produce a representative microstructure. DSC or DTA can
also be used for study of undercooling at the higher cooling rates (50CC/
min to 120°C/min) that more accurately represent the behavior of actual
small-scale solder joints. Collect data in a manner similar to that for
melting points.
4
Wetting Balance Test
2.0 Wetting Balance Test
Evaluate wetting behavior of solders and component leads.
2.1 Equipment
Commercial wetting balance tester or equivalent.
2.2 Specimens
The type of specimens depends on whether the solder and flux
combination is to be evaluated and compared with other combinations,
or whether component leads with different surface finishes are to be
evaluated for a fixed solder/flux combination. In the NCMS project,
various lead-free solders were compared using easily-solderable copper
wires. (Oxygen-free high-conductivity (OFHC) bare copper wire was
used, 25 mm long (1 in.), and 0.5 mm in diameter (0.02 in., 24 gauge).)
For testing of various component leads with different surface finishes,
test the specimens with a fixed solder/flux combination as specified
by J-STD-002 with the solder temperature as specified in Section 2.4.1.
2.3 Clean
Clean the OFHC copper wires per Section 3.2.4 of J-STD-002. Store the
specimen in anhydrous alcohol to prevent further oxidation prior to use.
2.4 Wetting Balance Test
(Per Sections 4.3.1 or 4.3.2 of J-STD-002.)
2.4.1 Test Temperatures
Two temperatures are suggested: (1) Liquidus +50°C (test
temperature in J-STD-002 for tin-lead eutectic is -50 °C higher
than its eutectic temperature); (2) Liquidus + 30°C (to be closer
to actual practice for lead-free alloys with most processes).
(Both temperatures allow reasonable evaluation of the
different assembly processes.)
Note: This procedure can also be used to study the wetting
characteristics of surfaces other than OFHC copper.
5
Test Procedures for Developing Solder Data
3.0 Tensile Specimen Fabrication
The specimen preparation procedures are intended to duplicate
the structure (grain size and orientation) in the actual solder joints.
Thus, specimens that are larger than joints should be cooled more
rapidly to obtain equivalent properties. Measurements of
microstructure, microhardness, and crystallographic texture
measurements help to establish this equivalence.
3.1 Uniaxial (Bulk Solder) Tensile Specimens
3.1.1 Dimensions
Gauge Length = 31.8 mm (1.250 ± 0.005 in.);
Diameter = 3.81 mm (0.150 ± 0.005 in.).
3.1.2 Procedure
3.1 .2.1 Preheat molds (aluminum molds for Sn-Bi alloys to enhance casting
properties and no mold release needed; titanium molds for high tin
alloys with graphite powder as a lubricant) to 150°C above liquidus
temperature of alloy.
3.1 .2.2 Maintain solder melt at 100°C above liquidus temperature with argon
shield over liquid solder to reduce dross buildup. The argon flow
should be very slow to avoid agitation.
3.1 .2.3 Allow 20 s between removing molds from oven and pouring the cast
(bottom fill). Stir the melt immediately prior to pouring. Pour slowly
and continuously until solder reaches top of mold, 5-7 s pour time.
Do not allow bridging between bars at top of mold.
3.1 .2.4 Immediately after pouring, quench molds in tap water by controlled
filling of quench tank. Water should fill tank in 60 s (using aluminum
molds) for complete solder solidification. Consider using an
intermediate tier in the quench tank to allow for a 10 s water level
dwell at the mold midpoint during filling. Run water until molds are
completely cooled, and remove castings from mold.
6
Tensile Specimen Fabrication
3.1 .2.5 Typical post-solidification aging: Anneal castings for 16 h at a
temperature two-thirds of the alloy's solidus temperature. Report
hardness data or other unusual characteristics of the specimens
(e.g., microstructure). Allow 24 h at room temperature for stress
relief before performing tests.
3.2 Lap Shear Specimens
3.2.1 Materials and Dimensions
Lap shear bar material: AISI type 1045 carbon steel with 0.0127 mmCu or Ni plating. Solder joint volume: 6.4 x 3.2 x 0.13 mm (width x
length x thickness).
3.2.2 Procedure
3.2.2.1 Pre-tin joint ends of lap shear bars with solder alloy. Clean with
alcohol.
3.2.2.2 Flux the pre-tinned ends with a standard activated rosin flux (such as
that listed in J-STD-002B), and align lap shear bars in a spring-loaded
fixture to maintain about a 0.013 mm (0.0005 in.) joint gap during
soldering.
3.2.2.3 Use electric hot tweezers to heat joint area to reflow temperature
(operation time 5-7 s).
3.2.2.4 Feed new solder into joint from one side to minimize voids in joint.
Allow to cool, and remove from fixture.
3.2.2.5 Remove solder joint fillets using a fine jeweler's file.
3.2.2.6 Typical post-solidification aging: Anneal for 16 h at a temperature
near two-thirds of the alloy's solidus temperature, and temper 24 h
at room temperature for stress relief before performing tests.
7
IV. ig Solder Data
4.0 Tensile Test Procedure
4.1 Procedure
Follow ASTM procedure E8.
4.2 Specimens
Tensile bar: per Section 3.1; Lap shear: per Section 3.2.
4.3 Test Parameters
Crosshead speed: To achieve strain rates between 10_3
/s and 10-4
/s,
although the full range of lO^/s (creep) to 10_2
/s is of interest.
Temperatures: -50 to 150°C, in 25°C steps (9 tests).
4.4 Evaluation
Measure strain with an extensometer; continue up to 20% before
stopping. Discontinue test if the load drops below ~ 2 kg. Measure
load as a function of displacement. Report elastic modulus, 0.2% offset
yield stress, ultimate tensile stress, uniform elongation, total elongation,
strength coefficient, and work hardening exponent.
8
Creep Test Procedure
5.0 Creep Test Procedure
5.1 Procedure
Follow ASTM procedure El 39.
5.2 Specimens
Butt joint.
5.3 Test Parameters
The strain rate can be 10-7
/s. Also, the loading can be stopped
occasionally to monitor the strain decrease as the load drops to zero,
developing on a single specimen, data for a range of creep rates.
Temperatures: -50 to 150°C, in 25 °C steps (9 tests).
5.4 Evaluation and Reporting
Measure strain as a function of time. Calculate the strain rate.
9
res for Developing Solder Data
6.0 Metallography
Examination of microstructures including different phases in solder and
along interfaces.
6.1 Mount
Mount the specimen on edge in epoxy. Use a slow-hardening mounting
material (such as Buehler Epoxide Resin with separate hardener) to
prevent the mount from pulling away from the specimen. Pull a vacuum
for the first five minutes that the mounting material is hardening. Release
vacuum, and allow 6-8 hours for the mount to cure fully. Use disposable
mounts which are glued together.
6.2 Grinding
Follow the sequence of 240/400/600 grit paper (600 grit is soft on top. not
like the coarser grits). Do not use grit coarser than 240. Spend no more
than 2 minutes on each step. Run the wheel at 200-250 rpm.
6.3 Polishing
Follow the media size sequence, 15um/6um/ 1 um/0.25um. starting with
diamond suspension (such as Buehler Metadi Supreme), which is water
based and comes in spray bottles. Use with appropriate polishing wheel
preparation, such as Buehler Met Grip liners (thicker w/ sticky backing).
With 15 urn to 6 um grit, use Texmet 1000 hand cloth. With 1 um to 0.25 urn,
use Microcloth (thicker nap). Then, use colloidal silica on microcloth.
Spend no more than 2 minutes on each step. Run the wheel at 300 rpm.
6.4 Etching
Appropriate for alloy. Etching of the specimen in a colloidal suspension
of silica is often used. Also, consider a mixture of 10% HC1 in methanol
for a few seconds to reveal the microstructure of most lead-free alloys,
and a mixture of 4 parts glycerol, 1 part acetic acid, and 1 part nitric acid
at 80°C to reveal the intermetallic surface.
6.5 Photomicrographs
Magnification of approximately 320 x for microstructure and 1000 to
1600x (air or oil immersion with optical microscope, or higher with a
SEM) for intermetallic compounds. Use scanning electron microscopy/
energy dispersive microscopy, as required, to identify intermetallic
compounds or phases.
10
Measurement of Solid State Growth Rate
7.0 Measurement of Solid State IntermetallicCompound Growth Rate (by dip method)
7.1 Equipment
One or more air furnaces having a maximum temperature of at least
250°C, controllable to ±1°C
7.2 Substrates
Metal substrates, such as copper, nickel, or palladium.
7.2.1 Copper Substrate
Try polished, OFHC copper sheets with solder coating (6.4 x 6.4 x
1.6 mm).
7.2.2 Coated Substrates
Coated substrates such as Au + Ni over Fe-Ni-Co alloy (Kovar™) as
well as Pd, Ni + Pd, or Ni + Pd + Au over Fe-Ni-Co or Cu substrates
could be used. Starting thicknesses of the substrate and various
coatings must be established prior to testing.
7.3 Solders
Solder should be kept in a pot with a minimum volume of 60 cc. Maintain
the solder pot temperature at 40°C greater than the liquidus temperature
of the alloy.
7.4 Specimen Fabrication Procedure
7.4.1 Coat the optically-polished side of the specimen with a flux that
provides suitable wetting.
7.4.2 Immerse the tab into the solder edge-on with the polished surface on
the bottom.
7.4.3 Immerse the specimen in the bath for 5 s.
7.4.4 Remove the specimen from the solder bath by maintaining the bottom,
polished surface parallel to the surface of the bath.
11
res for Developing Solder Data
7.4.5 Allow the specimen to cool.
7.4.6 Remove the flux residues by a suitable cleaning process.
7.5 Thermal Aging Procedures
7.5.1 Thermal aging is used to accelerate the solid-state growth of the
reaction (intermetallic compound) layer between the solder and the
substrate or solderable finish. Measurements are reported in terms
of thickness and intermetallic growth rate as functions of time,
temperature, and composition.
7.5.2 Aging temperature to be used per each of the solder compositions, per
aging times of 0. 10, 40, and 100 days, are listed below. Care must be
taken not to exceed the solidus temperature of the solder composition.
Alloy Composition Aging Temperatures (°C)
lOOSn 135.170. 205
96Sn-4Cu 135. 170. 205
95Sn-5In 135. 170. 205
95Sn-5Bi 135. 170. 205
91Sn-4Cu-5In 135. 170. 205
90Sn-5In-5Bi 100. 135. 170
91Sn-4Cu-5Bi 135. 170. 205
93Sn-2Cu-2.5Bi-2.5In 135.170. 205
86Sn-4Cu-5In-5Bi 100. 135. 170
76Sn-4Cu-10In-10Bi 100. 135. 170
7.6 Data Analysis
7.6.1 An optical or SEM micrograph should be taken of the intermetallic
compound layer at each of four different locations along the
solder/ substrate interface, using a minimum magnification of lOOOx
(1600 to 2000x is optimal). Because thickness measurements will
be determined from these micrographs, a magnification calibration
should be provided with each set of data. Figure 1 shows a
representative micrograph.
12
Figure 1. A typical lead-free solder joint structure after reflow. This SEM micrograph
(1800x) shows the (Ni,Cu)-Sn intermetallic reaction layer between Ni and solder and
Ag-Sn intermetallic rods crystallizedfrom Sn-Ag-Cu solder. Include a magnification
marker on the image, so the magnification calibration does not change with
enlargement in printing.
7.6.2 Measure and report the thickness of the intermetallic compound layer
at about ten equally-spaced positions along the interface (less for a very
uniform layer; as many as 40 for a layer with very uneven thickness).
Should more than one layer be present, measure the total layer
thickness as well as the thickness of each layer.
L3
Test Procedures for Developing Solder Data
8.0 Measurement of Liquid-State IntermetallicCompound Growth Rate (by the hot plate method)
8.1 Equipment
Programmable hot plate with a maximum temperature setting of at least
300 °C.
8.2 Substrates
Metal substrates, such as copper, nickel, palladium. Dimensions need to
be established prior to reflow (i.e., 10x5x 1 mm).
8.2.1 Copper Substrate
Try polished OFHC sheets.
8.2.2 Coated Substrates
Coated substrates such as Au + Ni over Fe-Ni-Co alloy (Kovar™) as
well as Pd, Ni + Pd, or Ni + Pd + Au over Fe-Ni-Co or Cu substrates
could be used. Starting thicknesses of the substrate and various
coatings must be established prior to testing.
8.3 Solders
Solder to be provided in paste form, with 50% of volume being flux.
8.4 Flux
Activated rosin flux per J-STD-002.
8.5 Specimen Fabrication Procedure
8.5.1 Preheat the hot plate to the desired reflow temperature; reflow
temperatures of 10°C, 20 °C, 30 °C, and 40 °C above melting point
or liquidus of solders are preferred.
8.5.2 Monitor temperature with a dummy specimen equipped with a
fine-gage Type K thermocouple.
14
Measurement of Liquid-State Growth Rate
8.5.3 Place the substrate on the hot plate once the hot plate reaches the
reflow temperature.
8.5.4 Apply a drop of flux to break-up the oxide layer, and wait 30 s for
the substrate to equilibrate to the hot plate temperature. Record
temperature of the dummy specimen using the thermocouple.
8.5.5 Apply known amount of solder paste (same ratio of substrate surface
to solder volume for constant thickness).
8.5.6 Start is the time at which melting occurs. (Monitoring the thermocouple
in the dummy specimen will also provide the start time).
8.5.7 Specimens are removed from the hot plate at specified times (i.e.. 20 s.
40 s. 60 s. 2 min. 4 min, 8 min) and quenched on an iron block or similar
media.
8.5.8 Mount and polish the sample to reveal the intermetallic layer.
8.6 Data Analysis
8.6.1 An optical or SEM micrograph is taken of the intermetallic compound
layer at each of four different locations along the solder/ substrate
interface, using a nominal magnification of 1600 to 2000 x. Because
thickness measurements will be determined from these micrographs,
a magnification calibration should be provided with each set of data.
8.6.2 Measure the intermetallic compound layer thickness at ten equally-spaced
positions along the interface. Should more than one layer be present,
measure the total layer thickness as well as the thickness of each
layer. A total of 40 data points shall be obtained for the intermetallic
compound layer thickness for each aging condition.
8.6.3 Plot time versus total intermetallic layer thickness for each reflow
temperature.
15
Test Procedures for Developing Solder Data
8.6.4 Fit the data to the equation x(t,T) = k tn
, where k = kQexp(-Q/RT),
using regression.
8.6.5 Refit the data to the average value of n and obtain k for each
temperature.
8.6.6 Arrehenius plot of k values (In k vs. 1/T) provide the pre-exponential
constant kQ and apparent activation energy Q.
16
Fracture Toughness
9.0 Fracture Toughness
9.1 Equipment
A materials testing machine equipped with a load cell range near 200 kg.
9.2 Specimens
A modified compact-tension specimen with a solder joint between two
machined brass halves plated with sufficient Cu (10.2 um) to withstand
reflow and aging conditions. Specimen dimensions are: approximately
32 mm long, 13 mm high, and 10 mm thick with the holes 25 mm from
the back of the specimen and 5.7 mm from the bottom.
9.2.1 Mask each half of the specimen except the surface to be soldered.
9.2.2 Coat the surface with a standard activated flux, and immerse it into a
pot of the appropriate solder at a temperature 40 °C above the liquidus
point to allow the formation of a solder coating of approximately
0.02 mm thick on each half.
9.2.3 Clamp the two halves together with an aluminum shim at the mouth of
the specimen to control the joint separation to 0.25 mm, and then recoat
the joint surfaces with flux.
9.2.4 Slowly immerse the clamped assembly into the molten solder.
9.2.5 Remove the specimen from the solder after capillary action fills the joint
(confirmed by flux being forced out the top of the specimen, but no
more than 30 s), and then allow to cool to room temperature.
9.2.6 Remove the clamp and spacer. Polish one side of the specimen to a
1 um finish using standard metallographic techniques; then lightly etch
and polish in a colloidal suspension of silica. This polishing technique
will confirm the thickness of the intermetallic layers and allow post-test
observation of crack location.
17
f Procedures for Developing Solder Data
9.3 Test Procedures and Parameters
9.3.1 Place the specimen into the load frame by inserting pins through the
specimen and corresponding holes in the grips.
9.3.2 Perform the tests at a displacement rate of 2.1 x 10~3 mm/s. Use a
high-resolution load cell (~ 200 kg capacity).
9.3.3 Record data through analog plots of load versus displacement (from
stroke motion of the load frame).
9.4 Evaluation
9.4.1 Quantify and analyze the fracture toughness as the value Kq (the
energy required to initiate a crack) per ASTM E399. (This assumes
that the results can be characterized by linear elastic fracture mechanics
and recognizes that E399 is intended for only monolithic materials.
Still, the results are useful for comparison ofjoints containing different
solder alloys.) The apparent fracture toughness is related to the fracture
load and the geometry of the specimen:
Where: - ^Kq ~ BW l/2
' f
P = load at fracture (kN)
B = specimen thickness (0.95 cm)
W = specimen width (2.54 cm)
a = crack length (1.74 cm)
f(a/w) = geometric factor (5.84).
a
W
9.4.2 Perform optical and SEM analyses on the cross section of the joint and
the fracture surfaces, respectively. Report the elemental composition of
the fracture surfaces in the SEM using energy dispersive x-ray analysis
and the void fraction in the solder joint (from image analysis of macro-
photos of the fracture surface).
18
Thermomechanical Fatigue Testing—Solder Joints
10.0 Thermomechanical Fatigue Testing —Simulated Solder Joints
10.1 Equipment
A servohydraulic 50 kN (10 kip) loadframe equipped with a 250 kg
(500 lb) load cell and a thermal cycling device capable of cycling
temperatures from -80 to 260 °C are required. Displacements are
measured with an extensometer. A computer controls all components
of the test and digital data collection including test-frame control electronics.
Compressed air is sent through the refrigeration unit, cooled to -80 °C,
and then heated to the required temperature by resistive heating coils
in the arm of the equipment. (Cooling the air first reduces the relative
humidity.) The cooled and reheated air is then directed to the test
specimen in the region of the solder joint. Temperature is monitored by
a thermocouple immediately adjacent to the specimen. The equipment
is controlled by an IEEE interface between the computer and the
equipment. A knife-edge extensometer is attached to the specimen
across the region containing the joints. The load frame is run in strain
control and the extensometer can measure, and control, the imposed
strain. The expansion and/or contraction of the specimen can be
accounted for by thermally cycling the specimen with only the
extensometer attached over the test temperature range. The 10%strain can then be added to the expansion/contraction of the specimen
to ensure that 10% strain is imposed on the specimen. The extent of
TMF is monitored by the load drop measured across the joints.
10.2 Specimens
The specimens are made from three Al boards having 9 solder
joints per side defined by machining and Cu electroplating techniques.
When assembled, the ends of the specimen are gripped and strain is
imposed along the axis of the specimen resulting in shear strain in each
of the joints.
10.2.1 Pre-tin the copper lands with the appropriate solder alloy.
10.2.2 Weigh out enough solder alloy to make joints that would have a
cylindrical shape 0.25 mm (0.010 in.) thick over the joint footprint.
10.2.3 Reflow the specimen in air, with an RJVIA flux, at temperatures 40°C
above the melting point or liquidus of the solder alloy.
19
lures for Developing Solder Data
10.3 Test Parameters
10.3.1 Temperature ranges for high-Sn lead-free solders, depending on
application: -55° to 150°C, -55° to 125°C, -40° to 125°C, or
0° to 100°C; (-55° to 90°C, -40° to 100°C, or 0° to 100°C for
lower temperature alloys such as Sn-58Bi).
10.3.2 Pre-strain: 10%.
10.3.3 The temperature /strain profile should follow a ramped wave with hold
times at the temperature /strain extremes. The hold times should be
3 minutes at each extreme with a deformation rate of 2.9 x 10_4/s
from one temperature extreme to another.
10.3.4 Data collection: It is useful to collect data on the resultant load,
temperature, strain applied, and the voltage across the joints.
The data should be collected and stored every 5 s during the cycle.
10.4 Evaluation
10.4.1 Plot hysteresis loops using the strain (or temperature) and the
resultant load for each cycle. The mechanical data collected during
the test can be described as a hysteresis loop during each cycle as a
plot of strain (or temperature) versus the resultant load. The data on
this plot can be described and summarized as: the maximum load on
the high-temperature portion of the cycle (high load), the minimumload on the low-temperature portion (low load), and the load to which
the specimen relaxes during the high-temperature hold portion of
the cycle (relax load). (No load relaxation was observed during the
low-temperature portion of the cycle.)
10.4.2 Metallography
1 0.4.2.1 Section the specimen and mount in epoxy.
1 0.4.2.2 Grind and polish down to a 1 um diamond finish.
1 0.4.2.3 Etch the specimen in a colloidal suspension of silica.
20
Thermomechanical Fatigue Test Using Vehicles
11.0 Thermomechanical Fatigue Test UsingPWB Test Vehicles
Follow the procedures in the latest version of IPC-9701.
21
Test Procedures for Developing Solder Data
12.0 Thermomechanical Fatigue Test Using Strips
This section is based on a paper by Pao, et al. See the reference in
the bibliography for more details.
12.1 Test Specimen
2 long strips (beams of the material couple of interest), joined by
solder at the ends. Aluminum and aluminum oxide (a CTE mismatch
of 14 ppm/°C) were used in the paper to introduce large strains,
but common materials used in electronics manufacturing might also
be evaluated.
12.2 Equipment
12.2.1 Strain Gages
Four high-temperature strain gages are mounted on the specimen
faces to measure the strains that occur during thermal cycling.
12.2.2 Wheatstone Bridge
A high-resolution bridge is used to improve the sensitivity to 20 fje I
MPa. The beam geometry, together with the high resolution bridge,
permits detection of stresses as low as 0.5 MPa in the specimens.
12.2.3 Thermal Cycling Chamber
Use a chamber capable of spanning the range of interest with
appropriate ramp rates. During thermal cycling, the CTE mismatch
between the two beams will introduce bending stresses.
12.3 Analysis
Calibrate the strain gages per the recommended procedure. Use the
equations in the paper by Pao to convert from strain in the beams to
stress in the solder. After the test, the specimen can be sectioned to
confirm the dimensions and to check for cracks. The data can then
be plotted and reported in conventional creep formats.
22
Liquid Solder Dissolution
13.0 Liquid Solder Dissolution
Experiments to determine the extent of dissolution of a substrate by
molten solder.
13.1 Equipment
Two solder baths with a capacity of 1 kg (2 lb) each. Digital image-
analysis system to measure changes in the substrate thickness.
13.2 Specimens
13.2.1 Specimen Materials
Metal substrates, such as copper, nickel, palladium.
13.2.1.1 Copper Substrate
Polished, oxygen-free, high-conductivity copper (OFHC) sheets.
13.2.1.2 Coated Substrates
Coated substrates such as Au + Ni over Fe-Ni-Co alloy (Kovar™)
as well as Ag, Pd, Ni + Pd, or Ni + Pd + Au over Fe-Ni-Co or Cusubstrates could be used. Starting thicknesses of the substrate and
various coatings must be established prior to testing.
13.2.2 Specimen Shape
13.2.2.1 Planar coupons measuring approximately 6.4 x 6.4 x 1.6 mm are
used for the tests.
13.2.2.2 Cylindrical specimens of wire, 0.68 mm (0.031 in.) in diameter.
13.2.3 Specimen Preparation
13.2.3.1 Degrease the substrates in trichloroethane (or equivalent), etch in
a 1: 1 solution of HC1 and water for 30 s, rinse to remove any acid
residues, and dry.
23
;edures for Developing Solder Data
1 3.2.3.2 Wrap the plate and wire specimens around their respective
perimeters with Kapton™ tape, leaving a 12 mm (0.5 in.) wide
surface "band" around the specimen, extending from the bottom
edge.
1 3.2.3.3 Coat the exposed regions of the plate and wire specimens with a
rosin-based, mildly-activated flux.
13.2.4 Solders
Sn-3.9Ag-0.6Cu (Tmelt
= 217-220°C);
Sn-3.5Ag (Tonset
= 221°C);
Sn-0.7Cu (Teut
= 227°C);
Sn-37Pb (Teut
= 183°C).
13.3 Test Procedure/Parameters
The immersion-time parameters are 5, 10, 20, 50, 75, and 100 s; three
solder pot temperatures are used per alloy; selected values are shown in
the table below.
Pot Temperatures (°C)
Solders Pot #1 Pot #2 Pot #3
Sn-3.9Ag-0.6Cu 240 255 270
Sn-3.5Ag 240 255 270
Sn-0.7Cu 250 265 280
63Sn-37Pb 205 220 235
13.4 Evaluation
13.4.1 Thickness Measurement of Plated Specimens
13.4.1.1 Measure 20 thicknesses on each of the non-immersed and immersed
segments of the coupons, and describe the results by a mean (x)
and ± one standard deviation (s) as the error term.
24
Liquid Solder Dissolution
Determine the dissolution distance by subtracting the thickness in
the immersed area from the original thickness of the plate; then,
dividing that value by two (2) so as to represent a single surface.
Determine the error term for the difference as follows:
(1) A fractional error is defined as s/x for each of the data sets
from the immersed and non-immersed regions. (2) The fractional
errors are added together, the value of which is designated as the
"fractional error for the difference." (3) This "fractional error
for the difference" is multiplied by the dissolution distance to
arrive at the ± error term. (4) The two data sets per each of
the time/temperature conditions are combined by combining the
immersed plate thickness data from Trial #1 with the same data
from Trial #2.
13.4.1.2 The same procedure is performed on the plate thickness of the
non-immersed regions. The values of x and s are computed; the
overall dissolution being the difference between the two means
of the combined data (divided by 2 to represent one surface).
The error term is computed in the same manner as described above.
13.4.2 Thickness Measurement of Wire Specimens
13.4.2.1 Measure the thickness reduction of the wire specimens at eight
diametrical positions about the specimen footprint.
13.4.2.2 Use the same procedures for data analysis as outlined above for
the plate specimens.
13.4.1.1.1
13.4.1.1.2
25
Test Procedures for Developing Solder Data
14.0 Acoustic Measurements of Elastic Constants
14.1 Equipment
An acoustic microscope.
14.2 Specimens
Cast the specimens into a cylindrical shape and machine to final
dimensions of 20 mm (0.8 in. long by 10 mm (0.4 in.)) in diameter.
14.3 Test Procedure/Parameters
The testing temperatures 23°C to 125°C with a minimum of 12 testing
temperatures between these extremes.
14.4 Evaluation
Determine the elastic constants as a function of temperature.
26
Ring-and-Plug Test
15.0 Ring-and-Plug Test
15.1 Equipment
A standard test load frame.
15.1.1 Test Fixture
Ring: Inner diameter = 3.175 mm (0.125 in.);
Outer diameter = 9.525 mm (0.375 in.);
Thickness = 3.175 mm (0.125 in.);
Plug: a solid cylinder having a diameter of 2.794 mm (0. 1 10 in.) and
length of 9.525 mm (0.375 in.), and effective joint gap (the difference
between outer diameter of the plug and inner diameter of the ring) is
0.19 mm (0.0075 in.).
15.2 Joint Fabrication
1 5.2.1 Degrease the Cu ring and plug by immersing in isopropyl alcohol for
2min.
15.2.2 Clean in fluoroboric acid (10% HBF in water) for 5 minutes at
room temperature with agitation (use caution in handling; also see
IPC-CS-70), rinse twice in distilled or deionized water, insert twice
in isopropyl alcohol, and air dry.
15.2.3 Coat with flux, J-STD-004, Type ROL1 (formerly RJVIA).
1 5.2.4 Insert the plug into the ring hole accompanied by four Cu wires of
0.18 mm (0.007 in.) diameter in order to maintain concentricity
between the plug and the ring.
1 5.2.5 Prepare a preform of the particular solder for testing from a short
length of wire by twisting the wire around the plug-ring intersection.
1 5.2.6 Place the assembled unit onto a stainless steel fixture and, in turn,
place it upon a hot plate held at 300 °C. Keep the assembly on the
hot plate for a duration of 15 s past the time at which the solder
preform is observed to have melted.
27
ledures for Developing Solder Data
15.2.7 Remove the assembly from the hot plate and allow to cool.
15.2.8 Cut off the protruding ends of the plug with a slow-speed diamond
saw, and polish the two faces flat.
15.3 Test Procedure/Parameters
15.3.1 Displacement rate =10 mm/min.
1 5.3.2 Run four tests per solder alloy composition.
15.4 Evaluation
Report the maximum load achieved. The results are represented as
the mean and plus-or-minus one standard deviation of the four tests.
Reference: ITRI Report 656.
28
Stress-Rupture ' Test .
16.0 Stress-Rupture Test
16.1 Test Specimens
Tensile bar: 31.7 mm (1.25 in.) gauge length, 3.8 mm (0.15 in.)
diameter; or lap shear: 6.4 mm x 3.175 mm x 0.127 mm (0.25 long x
0.125 wide x 0.005 in. thick), with AISI type 1045 carbon steel substrate
plated with 0.0125 mm (0.0005 in.) Cu or Ni.
16.2 Temperatures
-25°C, 25°C, 75°C, and 125°C.
16.3 Initial Stress
Low-melt alloys: 9 MPa (1250 psi) and 18 MPa (2500 psi);
High-melt alloys: 18 MPa (2500 psi) and 35 MPa (5000 psi).
16.4 Measure
Measure the time to failure and percent elongation at break. With the
assumption that there are no structural changes occurring in the material,
the slope of the stress-rupture line will remain constant. So, instead of
reporting the data as time to failure at a certain stress (e.g., 18 MPa,
2500 psi), report the stress to failure at certain times (10, 1,000, and
10,000 min.) for the comparison of the different solder materials.
29
edures for Developing Solder Data
17.0 Constitutive Behavior
17.1 Bulk Solder Specimens
Fabricate by casting the alloys in a split steel mold. Prior to casting,
heat the alloys 40°C above their melting point or liquidus, then pour
directly into the chilled mold. This will promote the formation of the
fine microstructure that is found in the small solder joints used in
electronic packages. Cut specimens from the cast ingots, and mount
in epoxy for metallographic observation. Hand polish on sequentially
finer nylon wheels using diamond paste; follow by polishing by a
colloidal suspension of silica on a vibratory polisher.
The mechanical response of the various solder alloys can be
determined through a series of isothermal uniaxial compression creep
tests. (The specimens can also be tested after isothermal aging to
coarsen the microstructures.) For test specimens, use right circular
cylinders 10 mm (0.4 in.) diameter by 20 mm (0.8 in.) in length. Use
a test machine fitted with an environmental chamber and a relatively
small (such as 55 kN) load cell. Test the alloys at -25°C, 25°C, 75°C,
and 125°C. Each specimen should be allowed to equilibrate in the
environmental chamber for 30 min. prior to testing. A compressive
prestrain of approximately one percent should be applied to the
specimens to ensure contact between the specimen and the platens.
Test in load control with stresses ranging from 10 to 80 MPa, applied
using a 1 s linear ramp. Platen displacement should be monitored as a
function of time with a digital oscilloscope with data stored on disk.
The ASCII-formatted strain-time data can be plotted to determine the
minimum strain rate for each stress and temperature. A plotting program
for engineering analysis should be used to fit the data according to the
Sherby-Dorn power-law creep equation:
e = A(crn /T)exp(-Q/RT)
Attempts might be made to fit the Pb-free solder alloy data to a sink
power-law equation. In past tests, the near-eutectic 60Sn-40Pb alloy
has shown power-law breakdown at temperatures lower than 20 °C,
and these data could be fit with the Garofalo sinh equation.
30
Bibliography
Joint Industry Standard: Guidelines for Chemical Handling Safety
in Printed Circuit Board Manufacturing. ANSI/IPC-CS-70. Rev. ed.
Northbrook: IPC.
Joint Industry Standard: Solderability Tests for Component Leads,
Terminations, Lugs, Terminals and Wires. ANSI/IPC/EIS J-STD-002A.
Rev. ed. Arlington: Electronic Industry Alliance or Northbrook: IPC.
Lead Free Solder Project. (A CDROM report.) Ann Arbor: National
Center for Manufacturing Sciences, 1999.
"Mechanical Testing and Evaluation." ASM Handbook, Vol. 8, Rev. ed.
Materials Park: ASM International.
Pao, Y.-H., et al. "Measurement of Mechanical Behavior of High Lead
Lead-Tin Solder Joints Subjected to Thermal Cycling." Journal of Electronic
Packaging, 114, June 1992, 135.
Standard Test Method for Conducting Creep, Creep Rupture, andStress Rupture Tests of Metallic Materials. ASTM E 139. Rev. ed.
West Conshohocken: ASTM.
Standard Test Method for Tension Testing of Metallic Materials [Metric].
ASTM E 8M. Rev. ed. West Conshohocken: ASTM.
31
